The invention relates to a PID controller having a parallel arranged P-element, I-element and D-element and in which the output signals from the P-element, I-element and D-element are added, and whose I-element has at least one weighting section and an integrator, in particular a digital controller.
Such a PID controller is known, for example, from the book by Heinz Unbehauen xe2x80x9cKlassiche Verfahren zur Analyse und Synthese linear kontinuierlicher Regelsystemexe2x80x9d Classical methods for analysis and synthesis of linear continuous control systems, publisher Vieweg 1982, pages 141 to 142. Figure 5.3.1 on page 142 shows a PID controller which is formed by connecting a P-element, an I-element and a D-element in parallel. The input signal of the PID controller is the control error formed from the reference variable and the actual value of the control variable. In the P-element, all that is done is to weight the control error by a factor Kp. The I-element comprises a weighting section, which multiplies the control error by a factor KI, and an integrator, which integrates the weighted control error. The D-element comprises a weighting section which multiplies the control error by a factor KD, and a differentiator which forms the time derivative of the weighted control error. In this PID controller, rapid changes in the reference variable, such as those which occur in the event of a sudden or a ramped change in the reference variable, lead to the integrator being charged up by the large control error. As soon as the actual value of the control variable has reached the reference variable, the integrator must be discharged again, which leads to the mathematical sign of the control error being reversed, that is to say to an overshoot. This is particularly true for control paths having a quasi-integral behavior. Control paths having a quasi-integral behavior are control paths which fundamentally have a PT1 behavior, but in which only the linear initial region is used. The step-function response of such a control path is described in Diagram 6 on page 29 of the book published by Mannesmann Rexroth GmbH, xe2x80x9cDer Hydraulik Trainerxe2x80x94Band 6: Hydrostatische Antriebe mit Sekundxc3xa4rregelungxe2x80x9d The hydraulic trainerxe2x80x94Volume 6: Hydrostatic drives with secondary control (RD 00 293/08.89). Control paths having a quasi-integral behavior include, for example, rotation-speed control of hydraulic secondary units and pressure regulation using hydraulic displacement pumps. Optimization of the control behavior of such control loops by lengthening the resetting time of the integrator admittedly reduces the tendency to oscillate in response to rapid changes in the reference variable, but causes a deterioration in the disturbance response of the control loop.
The invention is based on the object of providing a PID controller of the type mentioned initially, in which rapid changes in the reference variable lead only to a minor overshoot in the actual value of the control variable, that is to say of improving the reference behavior without the disturbance response being made worse.
This object is achieved by the invention wherein the time derivative (dy/dt) of the actual value of the control variable is fed to the I-element (e.g., 6 amd 7) with a negative mathematical sign. After a rapid change in the control error, which is caused by a rapid change in the reference variable, the invention reduces the signal fed to the integrator of the PID controller while the control error is subsequently being reduced. The PID controller according to the invention can be produced in a particularly simple form as a digital controller.